Porous metal materials are widely used in mechanical parts and various other industrial fields, and have been manufactured by a variety of methods. In the past, the manufacturing method involved molding and sintering a metal powder by raw material packing and sintering, powder compressing and sintering, or another such method. In recent years it has become more common to employ a manufacturing method in which metal is deposited by electroless plating, electroplating, vapor phase plating, or another such plating method on the surface of a three-dimensionally reticulated sponge-like foam skeleton of urethane or the like, or a manufacturing method in which a foam is immersed in a slurry in which a metal powder has been dispersed. Furthermore, a method for forming a felt-like nonwoven cloth in which metal fibers are irregularly intermingled (see Japanese Laid-Open Patent Application 56-88266), a method for sintering and calendering a wire aggregate of stainless steel (see Japanese Laid-Open Patent Application 4-165006), and the like have been investigated.
Meanwhile, the electrode sheet of a cell is generally constructed such that one of these porous metal materials is used as a core, and this porous metal material is packed with a positive or negative active material. As mentioned above, there is a wide range of porous metal materials that can be used as the core of an electrode sheet in a cell in which a chemical reaction occurs, but of these, porous metal foams having a three-dimensionally reticulated structure have generally been employed in recent years. These porous metal foams are produced by electroplating a sponge-like foam, and those used primarily as cores of electrode sheets in nickel-cadmium cells, nickel-hydrogen storage cells, and other such secondary cells are porous metal foams comprising, for example, a sponge-like metal (such as nickel) 1 in which numerous pores 2 have been formed, as shown in FIG. 18. The metal skeleton of these porous metal foams has a sponge-like three-dimensionally reticulated structure, so the porosity can be as high as 98%, which means that compared to conventional porous metal materials, the specific surface area is higher and the air permeation resistance is lower. Furthermore, the pressure loss is minute, and the product can be in any shape desired.
These porous metal foams are manufactured by the method given in Japanese Patent Publication 57-39317. Specifically, conductivity is imparted to a sponge-like foam such as a polyurethane sheet with a three-dimensionally reticulated structure by impregnation with carbon or another conductive paint, electroless plating, or another such means, after which a metal is deposited by plating onto the surface of the foam skeleton, and this product is heated to burn away only the sponge-like foam, which yields a porous metal foam.
However, with a cell in which the above-mentioned porous metal foam is used as a core for an electrode sheet, if the pores 2 in FIG. 18 are made larger in an effort to increase the amount of slurry-like active material with a relatively high viscosity packed per unit of volume, because the active material in the center of the pores 2, which is not in direct contact with the reticulated metal 1, does not play a major role in charging and discharging, this can lead to a decrease in the utilization rate of the active material, so there is no improvement in the discharge characteristics per unit of cell volume. Conversely, if the shape is such that the pores 2 are smaller and the porosity is higher, the amount in which the active material is packed will be smaller, electrical resistance will increase, and not enough current will flow. Consequently, good performance is not obtained when a cell in which this porous metal foam is used as an electrode sheet core is used in applications that demand a large flow of current, such as electric cars, electric tools, electric lawnmowers, and so on.
Furthermore, since the manufacture of a porous metal foam involves the use of plating, costs are incurred in related equipment and waste liquid disposal, and higher electrical consumption also drives up the cost. Also, because it is difficult to control the plating conditions, the plating rate cannot be increased, so no improvement in productivity can be achieved. All of this means that a porous metal foam is expensive, and this also makes a porous metal foam unsuitable as an electrode sheet core for electric car cells, which make use of numerous electrode sheets.
An object of the present invention is to solve the above problems encountered in the past, and provide a porous metal material that can be manufactured simply and at a low cost, and that allows the availability of active material to be increased, and to provide a method for manufacturing this porous metal material.